In the making of steel, metallurgical waste dust and sludge is created and collected from various sources. For example, in making of steel by electric arc furnace (EAF), EAF dust is generated that is typically collected in a baghouse or an electrostatic precipitator. Thirty or forty pounds of such EAF dust may be created for each ton of steel produced by EAF process. Similarly, in making steel by basic oxygen furnace (BOF), BOF dust is generated that is typically collected in a baghouse or an electrostatic precipitator in similar quantities. Cooling of the steel during processing using water also produces mill scale in relatively large quantities typically as sludge. In downstream processing in steelmaking, there may also be dust and sludge, such as galvanizing dust and sludge, from galvanizing and other hot dip coating, and spent pickle liquor from cleaning rust from intermediate steel products in pickling lines. In upstream processing, there may also be DRI dust or fines generated in the process of forming DRI from iron ore for use as a starting material in steelmaking, taconite tailings from the beneficiation of taconite to form taconite pellets used as a starting material, and blast furnace slag in making of pig iron in a blast furnace. There may also be fly ash collected in the stacks and flues in both upstream and downstream processes as well as the making of the steel itself.
These steelmaking dusts and sludges are high in iron content, but difficult to recycle or reclaim economically. EAF dust, for example, can be up to 50% by weight iron largely as iron oxide, but may also contains up to 30% by weight zinc and smaller quantities of calcium, magnesium, manganese, lead, cadmium and other metallic elements typically as simple and complex oxides. Because of the high levels of zinc, cadmium and lead, EAF dust, in particular, has been listed as a hazardous waste (KO61) in 40 C.F.R. §261.32 under the Resource Conservation and Recovery Act (“RCRA”), 42 U.S.C. §6901 et seq., requiring specific record keeping and particular handling and processing costs in disposal or recycling. While EAF dust has been subject to stabilization processes and disposed in landfills at considerable cost, one strategy has been to process the EAF dust in a BOF furnace into BOF dust and sludge that contains lower levels of zinc, cadmium and lead. See U.S. Pat. Nos. 6,562,096 and 6,562,101. BOF dust and sludge can be reused directly in steelmaking at some locations, or processed in an induction furnace to produce hot metal or pig iron (See U.S. Pat. No. 6,831,939); however, much BOF dust and sludge has been disposed as waste in land fills.
In another strategy, EAF dust is processed through a tunnel kiln to vaporize and oxidize the dust to recover high purity zinc oxide, and then the low zinc EAF dust is disposed of in land fills. See U.S. Pat. No. 6,682,586. In this process, lead and cadmium in the EAF can be halogenated and vaporized, and volatized halogens of lead and cadmium collected in the baghouse for recovery.
Another approach is to dissolve the EAF dust in nitric acid solution to form nearly complete dissolution of the iron, zinc, cadmium, copper, magnesium, calcium, manganese and lead. See U.S. Pat. No. 5,912,402. Iron is precipitated from the solution by raising the pH and/or by elevating the temperature. Cadmium, copper and lead are then removed in an electrolytic cell with copper and cadmium collected on the cathode and lead collected on the anode. Then calcium nitrate is removed by leaching from a filtrate, and the resulting residue treated with metal amine complexing agents such as ammonium carbonate, ammonium hydroxide, or similar agents to recover the zinc, leaving manganese and magnesium to be separated by acid. This approach resulted in separate recovery of various constituent metals in EAF dust, but has proved expensive and resulted in ancillary environmental concerns with the acids used.
A common metallurgical waste is mill scale, which is ubiquitous in steelmaking. Mill scale includes various forms of iron oxide formed at the surface of steel by oxidation from the surrounding atmosphere. See The Making, Shaping and Treating of Steel, at 946-947 (9th Ed. 1971). Mill scale is formed during heating, hot working and cooling of steel slabs, steel strip, blooms, and billets, as well as most other types of intermediate and finished steel products. The presence of such mill scale is particularly objectionable on the intermediate product to be further processed. For example, such scale typically must be removed and a clean steel surface provided if satisfactory results are to be obtained from the hot rolling of sheet or strip involving reduction or deformation of the steel. Similarly, if the steel sheet is for hot or cold drawing applications, the mill scale is removed as its presence on the steel surface tends to shorten die life, cause irregular and defective drawing conditions, and cause surface defects on the finish product. Scale is also removed if the sheet or strip is to be processed with a hot dip coating to permit proper alloying and adherence of the metallic coating, and satisfactory adherence when non-metallic coatings or paints are to be applied. Even where not a hazardous waste, mill scale such as BOF dust and sludge has been typically disposed of in land fills at considerable cost.
Additionally, other sources of iron-containing waste materials are available. In certain regions, iron-containing mine waste, such as wash-ore tailings and red ore tailings may be available for recovery of iron. Although there are considerable iron units in mill scale and similar metallurgical waste, there has not been available a commercially practical way of reclaiming or recycling of metallurgical waste.
One prior approach commonly used in the disposal of mill scale and similar metallurgical wastes in steelmaking was to “stabilize” or “capture” the waste material in a generally non-leachable form, typically with a basic material, such as lime or cement. Such stabilized waste materials subsequently are buried in designated waste landfills.
In the past, raw materials containing large amounts of FeO have been a problem in solid state reduction processes in hearth furnaces. In such previous processes, FeO melted before being reduced, called “smelting reduction,” producing a highly fluid and aggressive slag. Even the melted FeO that reacted with carbon in reduction caused damage and erosion of refractory hearths. Moreover, large amounts of FeO typically remained in the slag reducing the effectiveness of the reduction process. U.S. Pat. No. 6,630,010 to Ito, et al. discloses a method of reducing metallic waste containing iron oxides describing a complex two step heating process to reduce FeO.
The need for a commercially practical way of reclaiming or recycling iron from mill scale and similar metallurgical waste has been emphasized by public awareness of environmental issues in solid waste, the decreasing availability of landfill areas, and the continuing awareness of the earth's mineral resources. Additionally, economic pressures and the tightening of competition for uses of the earth's natural resources have increased. Further, federal and state regulations regarding the use of the earth's natural resources and the disposal of waste materials have become more encompassing and more restrictive. As a result, there remains a need for reducing mill scale and similar metallurgical waste, and reclaiming and recycling of iron units where economically possible.
What is disclosed is a practical and economical way of disposing of mill scale and similar metallurgical waste in steelmaking, while reclaiming valuable iron units from these metallurgical wastes. It provides as a by-product nodular reduced metallic iron (NRI) that can be used as a substitute for scrap in economically making steel by EAF process.
A method of recovering metallic iron from iron-bearing metallurgical waste in steelmaking is disclosed, including steps of:
(a) providing an iron-bearing metallurgical waste containing more than 55% by weight FeO and FeO equivalent and a particle size of at least 80% less than 10 mesh,
(b) mixing the iron-bearing metallurgical waste with a carbonaceous material to form a reducible mixture where the carbonaceous material is between 80 and 110% of the stoichiometric amount needed to reduce the iron-bearing waste to metallic iron, and as needed additions to provide a silica content between 0.8 and 8% by weight and a ratio of CaO/SiO2 between 1.4 and 1.8,
(c) forming agglomerates of the reducible mixture over a hearth material layer to protect the hearth,
(d) heating the agglomerates to a higher temperature above the melting point of iron to form nodules of metallic iron and slag material from the agglomerates by melting.
The carbonaceous material in the reducible mixture may be between 85 and 100% of the stoichiometric amount needed to reduce the iron-bearing waste to metallic iron.
The iron-bearing metallurgical waste is typically mill scale. Mixed with the mill scale may be iron-bearing metallurgical waste selected from the group of DRI fines, processed EAF dust, BOF sludge, blast furnace dust, wash ore tailings, red ore tailings, and mixtures thereof. The mixture is particular advantageous when the availability of mill scale is in short supply.
The mill scale and similar iron-bearing metallurgical waste may be provided of at least 80% less than 14 mesh. Additionally, the method may further include the step of mechanically reducing particle size of the iron-bearing metallurgical waste to at least 80% less than 14 mesh. The iron-bearing metallurgical waste may be mixed with less than 8% by weight lime and less than 4% by weight fluorspar.
In the present method, the silica source may be at least in part from the iron-bearing metallurgical waste. The silica source may be at least partially selected from the group consisting of sand, EAF slag, LMF slag, BOF slag, fly ash, taconite tailings, wash ore tailings, floatation tailings, DRI fines, blast furnace slag, and mixtures thereof.
The hearth material layer may be a carbonaceous material selected from the group consisting of PRB coal/char, bituminous coal, anthracite and coke of more than 80% between 100 mesh and 3 mesh.
In the present method during the heating step the agglomerate may be heated to greater than 2450° F.
The method may include the additional step after forming the agglomerates and before heating the agglomerates of providing an overlayer of coarse carbonaceous material of between 6 mesh and 1 inch over the agglomerates. Alternatively, the overlayer of coarse carbonaceous material may be between 6 mesh and ⅝ inch. The overlayer of coarse carbonaceous material may be between about 0.5 lb/ft2 (2.44 kg/m2) and about 1.25 lb/ft2 (6.10 kg/m2).
The iron-bearing metallurgical waste may be mixed with a combination of high volatile carbonaceous material selected from the group consisting of sub-bituminous coal and PRB coal and low volatile carbonaceous material selected from the group consisting of anthracite, bituminous coal, coke breeze, coke, and char as the carbonaceous material.
The step of forming the agglomerates on the hearth may involve first forming agglomerates of the reducible mixture and then placing the agglomerates on the hearth material layer.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. A more complete understanding of the invention and its advantages will become apparent by referring to the following detailed description and claims in conjunction with the accompanying drawings.